1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and its manufacturing method.
2. Description of the Related Art
The TN (twisted nematic) mode is one of the current modes used for LCD devices. In this mode, an electric field vertical to the surface of the substrate is used to orient the liquid crystal molecule director (the molecular major axis). By doing this, the optical transmittance is controlled so that an image can be displayed on the LCD panel. This is a common type (hereafter called a vertical electric field driver-type) of LCD device.
Since, with the vertical electric field driver-type LCD, the director is oriented to be vertical to the surface of the substrate when the electric field is applied, the refractive index changes depending on the viewing angle. Accordingly, the vertical electric field driver-type LCD is not suitable when a wide viewing angle is needed.
There are also LCD devices where the liquid crystal director is oriented parallel to the surface of the substrate. These are devices where the electric field functions in a direction parallel to the surface of the substrate so that the director can rotate in a plane parallel to the surface of the substrate. Through this, the optical transmittance is controlled, and an image is displayed. This type (hereafter called a lateral electric field driver-type) of LCD device has only just been developed in recent years. With the lateral electric field driver-type LCD, because the change in refractive index due to the viewing angle is remarkably small, a high quality display can be obtained.
An example of this type of lateral electric field driver-type LCD is shown in FIGS. 1 to 3. FIG. 1 is a plan view of a vertical electric field driver-type LCD, FIG. 2 is a cross-sectional view of the LCD in FIG. 1 taken along the line JJ′, and FIG. 3 is a cross-sectional view of the LCD in FIG. 1 taken along the line KK′. The pixel shown in these diagrams is formed of the following elements: a data line 1, a scanning line 2, a thin film transistor (TFT) 3, a common electrode 4 and a pixel electrode 5. The scanning line 2 is connected to an external drive circuit (not shown in the figures). The TFT 3 is a switching device. The scanning line 2 and the common electrode 4 are both structured on a substrate 10 where TFTs are fabricated (hereafter, called TFT substrate 10). The pixel electrode 5 and the data line 1 are structured on the scanning line 2 and the common electrode 4 via an interlayer insulation film 7. The pixel electrode 5 and the common electrode 4 are alternately positioned. These electrodes are covered with a protection/insulation film 8. On the protection/insulation film 8, an alignment layer 15 is laid and subjected to a rubbing treatment.
A black matrix 9 to shield light is structured in a matrix format on the underside of the opposite facing glass substrate 11. The primary and secondary colored layers 12 and 13, which are necessary for color display, are prepared on the black matrix 9. Each of the colored layers 12 and 13 are assigned to each pixel. Here, the above two colors represent two of the three primary colors: red, green, and blue. But, the one remaining colored layer is not shown in the figures.
On top of the primary and secondary colored layers 12, 13, an over-coating film 14 necessary to make the opposite facing substrate 11 flat is prepared. An alignment layer 16, which will be necessary to orient the liquid crystal 18, is laid on the over-coating film 14 and then subjected to a rubbing treatment. The rubbing treatment is performed in the direction opposite to that performed on top surface of the TFT substrate 10.
Next, liquid crystal 18 and spacers 17 are poured into the gap between the TFT substrate 10 and the opposite-facing substrate 11. The spacers 17 are randomly distributed throughout the area between them. The minimum distance between the two substrates determines the diameter of the spacers 17.
A polarizer film (not shown in the figures) is applied to the outer surface of the TFT substrate 10 where the electrode patterns have not been formed. This polarizer film is applied in a manner such that the transmission axis runs in the direction perpendicular to the direction of the rubbing. A polarizer film (also not shown in the figures) is applied to the outer surface of the opposite facing glass substrate 11 where there are no layered patterns. The transmission axis of the polarizer film on the opposite facing glass substrate 11 is perpendicular to the direction of the transmission axis of the polarizer film on the TFT substrate 10.
The LCD panel with the above structure is set up on a backlight and attached to a drive circuit.
In the above mentioned conventional LCD device, the liquid crystal poured into the narrow gap between the TFT substrate and the opposite facing substrate is normally oriented parallel to the direction that the rubbing treatment was performed on the alignment layers 15 and 16. As shown in FIG. 4, the liquid crystal molecules 20 surrounding each spacer 17 are oriented parallel to the surface of the spacer 17. In this case, when the screen is in normally black mode (i.e. the mode where no light can pass through when no voltage is applied), light permeates through the area where the liquid crystal molecules are lined up askew to the polarizer film absorption axis (for example, the liquid crystal molecules in region 21). Due to this, a leakage of light develops in the fan blade-shaped regions 21. In addition, the weak aligning force causes the alignment of the liquid crystal surrounding the spacers 17 to fall into disorder. When this happens, the amount of leakage of light around the spacers 17 increases; subsequently, as shown in FIG. 5, a doughnut-shaped region 21 of leakage of light develops.
Furthermore, when the liquid crystal panel happens to be impacted, the spacer 17 becomes charged by the friction created from being scraped against the alignment layers on the TFT substrate and the opposite facing substrate respectively. Once this occurs, a radial electric field develops around the spacers 17. In this case, because the liquid crystal molecules 20 become aligned parallel to the electric field, fan blade-shaped regions 21 of leakage of light develop, as shown in FIG. 6.
At this point, when comparing the two cases where the spacer 17 is not charged as shown in FIG. 4 and where the spacer 17 has been charged up as shown in FIG. 6, it is apparent that the latter case gives larger radial areas of leakage of light 21.
This type of charging occurs when a certain pressure or impact, which happens to hit the LCD panel, causes spacers that are positioned in the opaque region of liquid crystal molecules (i.e., in the region of crystal molecules under the black matrix) to move and be strongly rubbed by the top surfaces of the alignment layers. This occurs easily since the gap at the opaque region (i.e., the region under the black matrix 9 and on the data line 1, the scanning line 2, the TFT 3, etc.) is narrower than the gap at the transparent regions, which widens the contact area of the spacer with either surface of the alignment layers. The wider contact area allows a conveyance of a strong force, which is caused by the certain pressure or impact being applied to the LCD panel, onto the spacer. This force can easily push and move the spacer out into a transparent region of liquid crystal molecules. An electrically charged spacer that has entered the transparent region increases the total amount of leakage of light, which in turn causes a deterioration of display quality. On the other hand, a spacer that is originally positioned within the transparent region of liquid crystal molecules is rarely charged electrically by this type of movement since the gap of the transparent region is wider.
As described above, when a certain pressure or impact is applied to the LCD panel, the spacer that is positioned within an opaque region can easily migrate to a transparent region and, especially when the LCD shows at all display area, an increase in leakage of light 21 becomes noticeable. Besides, when the distribution of the spacers 17 is not uniform, the display quality becomes distorted and a problem develops where the contrast decreases due to the leakage of light.